Kappa(河童, "river-child"), alternatively called Kawatarō (川太郎, "river-boy") or Kawako (川子, "river-child"), are legendary creatures found in Japanese folklore. In Shintō they are considered to be one of many suijin. A hair-covered variation of a Kappa is called a Hyōsube (ひょうすべ).
Kappa are similar to Finnish Näkki, Scandinavian/Germanic Näck/Neck, Czech Vodník and Scottish Kelpie in that all have been used to scare children of dangers lurking in waters.
Kappas are typically depicted as roughly humanoid in form, and about the size of a child. Their scaly, reptilian skin ranges in color from green to yellow or blue. Despite their reptilian features, Kappa are said to have apelike faces and scraggly green hair.
Kappas supposedly inhabit the ponds and rivers of Japan and have various features to aid them in this environment, such as webbed hands and feet. They are sometimes said to smell like fish, and they can certainly swim like them. The expression kappa-no-kawa-nagare ("a kappa drowning in a river") conveys the idea that even experts make mistakes.
Their most notable feature is an indentation on the top of their head that holds water called a sara; this is regarded as the source of their power. This cavity must be full whenever a kappa is away from the water; if it spills, the kappa will be unable to move. Although they are reported to inhabit all of Japan, they are often said to be particular to Saga Prefecture.
Japanese folklore states that the kappa can be very polite, and can be disabled before it attacks by bowing. The kappa would bow back, and spill the water, disabling itself. Kappa are usually seen as mischievous troublemakers. Their pranks range from the relatively innocent, such as loudly passing gas or looking up women's kimonos, to the malevolent, such as drowning people and animals or kidnapping children. When kappas attack people, they often pull the person's intestines out through their anus, in order to get to a mythical organ called the shirikodama, which is said to contain the human's soul. They are also said to love cucumbers, and people can sometimes escape from an aggressive kappa by throwing a cucumber in its direction, and run away while it eats the cucumber. If it is necessary to bathe in the water a kappa lives in, it is possible to throw a cucumber into the water before entering, after which the kappa will allow you to use its water without attacking you. If somebody defeats a kappa it may surrender, after which it will help with farming, or share medical knowledge. It is also possible to befriend a kappa by bringing it cucumbers, which will also cause it to help in those ways. The technique of bone setting was thought to have been originally taught to humans by a friendly kappa. Kappas also frequently challenge humans to sumo wrestling, which they are almost guaranteed to win at, unless the human gets them to spill the water in their head, after which the kappa is forced to retreat. It also has a type of sushi that contains cucumbers named after it, the kappamaki.
One possible explanation for these creatures is the giant Japanese salamander which is rumored to grow up to eight feet in length and lives only in clean, clear streams (which is unfortunately where people like to play).
Another is that it could be a new or prehistoric kind of amphibian, such as a temnospondyl.
It could also be a misinterpretation of a sea turtle, or perhaps an undiscovered species of Turtle. This could be likely, as some turtles are known to eat cucumbers.